1. Field of the Invention
This invention is directed to a method of reducing gaseous inclusions in a glass making process. The invention is particularly useful for reducing gaseous inclusions in high melting temperature or high strain point glasses, such as those that are used for glass substrates for flat panel display devices.
2. Technical Background
Liquid crystal displays (LCDs) are flat panel display devices that include flat glass substrates or sheets. The fusion process is a preferred technique used to produce sheets of glass used in LCDs because the fusion process produces sheets whose surfaces have superior flatness and smoothness compared to sheet produced by other methods. The fusion process is described, for example, in U.S. Pat. Nos. 3,338,696 and 3,682,609, the contents of which are incorporated herein by reference.
Typically, LCDs are of the amorphous silicon (α-Si) thin film transistor (TFT) or polycrystalline-silicon (ρ-Si or poly-Si) TFT type. Poly-Si has a much higher drive current and electron mobility, thereby increasing the response time of the pixels. Further, it is possible, using ρ-Si processing, to build the display drive circuitry directly on the glass substrate. By contrast, α-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques.
The evolution from α-Si to ρ-Si has presented a major challenge to use of a glass substrate. Poly-Si coatings require much higher processing temperatures than do α-Si, in the range of 600-700°. Thus, the glass substrate must be thermally stable at such temperatures. Thermal stability (i.e. thermal compaction or shrinkage) is dependent upon both the inherent viscous nature of a particular glass composition (as indicated by its strain point) and the thermal history of the glass sheet as determined by the manufacturing process. High temperature processing, such as required by poly-Si TFTs, may require long annealing times for the glass substrate to ensure low compaction, e.g. 5 hours at 600° C. These needs have driven glass manufacturers to search for higher melting point glasses. However, high melting point, high strain point glasses present several manufacturing challenges. To begin, the glass should be compatible with current manufacturing methods.
Conventional glass manufacturing processes for LCD glass typically begin by melting glass precursors—feed materials—in a melting furnace. Reactions which occur during this melting stage release gases which form bubbles (also referred to as seeds or blisters) in the glass melt. Seeds may also be generated by interstitial air trapped between particles of the feed materials. In any event, these gas bubbles must be removed in order to produce high quality glass. The removal of gaseous inclusions is generally accomplished by “fining” the glass. For clarity, gaseous inclusions formed as a result of the melting process, whether as reaction products or interstitial gases, will be referred to hereinafter as “seeds”.
A common method of fining a glass melt is by chemical fining. In chemical fining, a fining agent is introduced into the glass melt, such as by addition to the feed material. The fining agent is a multivalent oxide that is reduced (loses oxygen) at high temperatures, and is oxidized (recombines with oxygen) at low temperatures. Oxygen released by the fining agent may then diffuse into the seeds formed during the melting process causing seed growth. The buoyancy of the seeds is thereby increased, and they rise to the surface of the glass where the gas is released out of the melt. Ideally, it is desirable that the fining agent release oxygen late in the melting process, after most of the seeds have formed, thereby increasing the effectiveness of the fining agent. To that end, although large seeds may be eliminated in the melter, the glass typically undergoes additional fining in a fining vessel, where the temperature of the glass is increased above the melting temperature. The increase in temperature of the glass melt within the fining vessel reduces the viscosity of the glass, making it easier for seeds in the melt to rise to the surface of the glass, and an oxide fining agent will release oxygen to the melt to cause seed growth and assist with the seed removal process. Once the melt has been fined, it may be cooled and stirred to homogenize the melt, and thereafter formed, such as into a glass sheet, through any one of a variety of available forming methods known in the art.
Many glass manufacturing processes employ arsenic as a fining agent. Arsenic is among the highest temperature fining agents known, and, when added to the molten glass bath in the melter, it allows for O2 release from the glass melt at high temperatures (e.g., above 1450° C.). This high temperature O2 release, which aids in the removal of seeds during melting and in particular during the fining stages of glass production, coupled with a strong tendency for O2 absorption at lower conditioning temperatures (which aids in the collapse of any residual gaseous inclusions in the glass), results in a glass product essentially free of gaseous inclusions.
From an environmental point of view, it would be desirable to provide alternative methods of making glass, and particularly high melting point and strain point glasses typically employed in the manufacture of LCD glass, without having to employ arsenic as a fining agent. Arsenic-containing compounds are generally toxic, and processing of glass with arsenic results not only in manufacturing wastes that are expensive to process, but also creates disposal issues relative to the display device itself after the useful life of the device is exhausted. Unfortunately, many alternative fining agents typically release less oxygen, and/or at too low a temperature, and reabsorb too little O2 during the conditioning process relative to established fining agents such as arsenic, thereby limiting their fining and oxygen re-absorption capabilities. Thus, during the fining stage of the glass production process (i.e. while the glass is within the fining vessel), the fining agent may produce an insufficient quantity of oxygen to effectively fine the glass within the fining vessel.
It would therefore be beneficial to find a process that can reduce gaseous inclusions without the need for the use of toxic fining agents.